The present invention relates to a solid-state imaging device and a method for driving the same.
In a solid-state imaging device, a plurality of photoelectric transducers, like photodiodes, are arranged in a matrix fashion in the imaging area thereof. Examples of solid-state imaging devices include not only charge-coupled device (CCD) types, but also metal-oxide-semiconductor (MOS) types. As in the CCD-type imaging devices, blooming is also caused in the MOS-type devices.
Hereinafter, blooming will be described in detail with reference to FIGS. 18A and 18B. FIG. 18A schematically illustrates a cross section of a single pixel portion of the imaging device, where photodiodes and transistors are formed, near the surface of a semiconductor substrate. FIG. 18B illustrates distribution of potentials in that part of the semiconductor substrate shown in FIG. 18A.
As shown in FIG. 18A, a photoelectric transducer 3 and respective gate electrodes of a transfer gate 4 and a reset transistor 7 are formed within the pixel. The photoelectric transducer 3 within the pixel shown in FIG. 18A is electrically isolated from a transistor section, including photoelectric transducer and detection portion, within an adjacent pixel (not shown) by an impurity doped layer (i.e., a channel stop region) with a dopant concentration higher than that in the substrate, LOCOS or the like. In the pixel with such a configuration, signal charges (electric carriers), which have been created by the photoelectric transducer 3 responsive to incoming light, are transferred through the transfer gate 4 into a detection portion 5 and then discharged to a power supply VDD via the reset transistor 7.
However, suppose intense light continues to enter the photoelectric transducer 3 even during an interval in which the transfer gate 4 and the reset transistor 7 should be OFF. In such a situation, charges with a quantity exceeding the maximum quantity storable in the photoelectric transducer 3 are sometimes created to overflow from the photoelectric transducer 3 as shown in FIG. 18B. As a result, the overflowing charges pass through the transfer gate 4 and the channel stop region to flow into the detection portion or the photoelectric transducer within the adjacent pixel. This is a phenomenon similar to a phenomenon called xe2x80x9cbloomingxe2x80x9d in the field of CCD image sensors. As in CCD""s, if blooming is caused in an amplifying solid-state imaging device, then some white band-like or circular patterns, deteriorating the quality thereof, are observed in an image captured. In order to suppress such blooming, solid-state imaging devices, including various types of overflow drain structures in respective imaging areas within respective semiconductor substrates, have been developed.
However, if an overflow drain structure is specially provided for an imaging area within a semiconductor substrate, then the manufacturing process of the device is adversely complicated or it becomes harder to downsize each photoelectric transducer. In addition, a control signal should be specially produced and applied to operate the overflow drain.
An object of the present invention is providing a solid-state imaging device that can suppress blooming with a simplified structure and a method for driving the same.
A solid-state imaging device according to the present invention includes an imaging area including a plurality of pixels arranged in columns and rows, and peripheral circuitry for selecting at least one of the pixels. Each said pixel includes: a photoelectric transducer for creating electric charges by converting incoming light into electric energy and for storing the charges therein; means for storing the charges that have been read out from the photoelectric transducer; a transfer electrode, provided between the photoelectric transducer and the storage means, for reading out the charges from the photoelectric transducer to the storage means; an amplifier for sensing a variation in potential in the storage means; and a reset electrode for discharging the charges, which have been stored in the storage means, to a power supply, thereby resetting the potential in the storage means. The peripheral circuitry includes a selector for generating first and second control signals. The first control signal is applied to the transfer electrode to control the height of a first electrical barrier under the transfer electrode, while the second control signal is applied to the reset electrode to control the height of a second electrical barrier under the reset electrode. The selector includes a bank of multi-stage inverters operative upon the application of first and second power supply voltages thereto. The second power supply voltage is lower than the first power supply voltage. Some of the inverters on the last stage generate biased output signals to be supplied as the first and second control signals.
Another solid-state imaging device according to the present invention includes: an imaging area including a plurality of pixels arranged in columns and rows; and peripheral circuitry for selecting at least one of the pixels. Each said pixel includes: a photoelectric transducer for creating electric charges by converting incoming light into electric energy and for storing the charges therein; an amplifier for sensing a variation in potential in the photoelectric transducer; and a reset electrode for discharging the charges, which have been stored in the photoelectric transducer, to a power supply, thereby resetting the potential in the photoelectric transducer. The peripheral circuitry includes a selector for generating a control signal to be applied to the reset electrode to control the height of an electrical barrier under the reset electrode. The selector includes a bank of multi-stage inverters operative upon the application of first and second power supply voltages thereto. The second power supply voltage is lower than the first power supply voltage. Some of the inverters on the last stage generate a biased output signal to be supplied as the control signal.
Still another solid-state imaging device according to the present invention includes a plurality of pixels and peripheral circuitry for selecting at least one of the pixels. Each said pixel includes information storage for storing therein electric charges created by photoelectric conversion, and a reset device, responsive to a control signal, for varying the height of an electrical barrier existing between the information storage and a power supply, thereby discharging the electric charges, which have been stored in the information storage, to the power supply. The peripheral circuitry includes a selector for generating the control signal. The selector includes a bank of multi-stage inverters operative upon the application of first and second power supply voltages thereto. The second power supply voltage is lower than the first power supply voltage. Some of the inverters on the last stage generate a biased output to be supplied as the control signal to the reset device.
A method according to the present invention is adapted to drive an embodiment of the solid-state imaging device according to the present invention. The method includes the steps of: creating electric charges by photoelectric conversion to store the charges in the information storage; and increasing a level of the control signal, generated by the selector, like pulses to make the reset device reset the charges in the information storage.
Another method according to the present invention is adapted to drive another embodiment of the solid-state imaging device according to the present invention. The method includes the steps of: creating electric charges by photoelectric conversion to store the charges in the photoelectric transducer; increasing a level of the first control signal, generated by the selector, like pulses to make the reset device reset the charges existing between the transfer electrode and the reset device; and increasing a level of the second control signal, generated by the selector, like pulses to transfer the charges in the photoelectric transducer to between the transfer electrode and the reset device via the transfer electrode.
According to the present invention, an excessive quantity of charges, which have been created in a photoelectric transducer, can be discharged toward a first power supply VDD via a reset transistor, for example. As a result, the overflow of charges within a pixel into an adjacent pixel can be prevented or at least drastically reduced, thus suppressing the generation of blooming.